On the one hand, potassium magnesium sulfate (SOPM) is a very desirable fertilizer used in intensive agriculture, incorporating three agronomic elements: potassium, magnesium and sulfur. However, as a mined product, SOPM not only is found in diminishing quantities, but also has a chlorine content of one to several percent, which reduces its efficiency in agricultural applications. A synthetic approach allowing nearly chlorine-free SOPM would therefore ensure high quality supplies and intensive uses without the sterilizing effect of chloride on soils.
It is known to generate hydrochloric acid from the action of sulfuric acid on a chloride. At temperatures in the range between 100 and 160° C., the reaction of sulfuric acid with potassium chloride (potash) leads to hydrochloric acid and an acid potassium sulfate, KHSO4, as follows:H2SO4+KCl→KHSO4+HCl  (Equation I)
In order to achieve the complete substitution of potassium for both hydrogens on the sulfuric acid, much higher temperatures are required, in the range of 400° C., as well noted in the art (Mannheim process, Chemical Process Industries, R. N. Shreeve, McGraw-Hill, 3rd ed., 1967, p. 346). At this temperature, HCl is obtained together with potassium sulfate K2SO4 as follows:H2SO4+2KCl→K2SO4+2HCl  (Equation II)
As people in the art will appreciate, such a high temperature reaction leads to severe corrosion problems, difficult heat transfer and large energy consumption.
Therefore, there is a need in the art for a method to overcome the above-mentioned shortcomings.